Mass spectrometers often employ multipole ion guides to focus and confine ions as they are transported along a path from the ionization source to the mass analyzer. Ion guides generally include a plurality of elongated electrodes (sometimes referred to as rod electrodes) to which oscillatory voltages are applied to establish a radially confining field. In addition to the ion transport function, ion guides may be employed for the radial confinement of ions in a collision cell, in which the internal volume of the ion guide is pressurized with collision gas, and ions entering the ion guide undergo fragmentation via the collision-induced dissociation mechanism.
When ion guides are located in relatively high-pressure regions of the mass spectrometer, such as in chambers adjacent the ionization source or within a collision cell, the initial axial velocities of the incoming ions are sharply reduced due to collisions between the ions and background/collision gas. This reduction in ion axial velocity results in a higher residence time within the interior of the ion guide, which may adversely affect instrument performance. More specifically, prolonged ion residence times within the ion guide may reduce sample throughput, decrease sensitivity, and impose limits on various aspects of operation. In the example of triple quadrupole mass spectrometers operated in multiple reaction monitoring (MRM) mode, slowing of ions within the collision cell or upstream ion guides will lengthen the required dwell time at each precursor-product ion transition, thereby constraining the number of different transitions that may be monitored per unit time.
In order to increase the rate at which ions are axially transported through ion guides, it is known to establish a static axial field along part or all of the ion guide length to urge ions in the direction of the ion guide exit. Various structures and methods have been disclosed in the prior art for producing an axial field of this type (see, e.g. U.S. Pat. Nos. 5,847,386; 6,111,250; 6,713,757; 7,067,802; and 7,675,031, which are hereby fully incorporated by reference herein). However, these structures and methods tend to cause distortion of the radially-confining oscillatory (e.g., radio-frequency (RF)) field, which may result in defocusing of the ion beam and consequent reduction in transmission efficiency. Applicant believes that there is a need in the mass spectrometry art for an ion guide having structures for establishing an axial field that avoids the radial-field distortion effects present in prior art devices.